The citric acid cycle (TCA cycle) was discovered by Hans Krebs in 1937. It is a series of reactions in the mitochondria that oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins to carbon dioxide, producing reduced co-enzymes that drive ATP production. The cycle consists of 8 steps catalyzed by different enzymes, with oxidative decarboxylation steps producing NADH and FADH2. The cycle plays important biosynthetic and anaplerotic roles in addition to its main role in energy production. Key control points ensure regulation of flux through the cycle.

1. (TCA CYCLE & ITS REGULATION)
Submitted To:-
Dr. Joginder Singh Duhan
Assistant Professor
(Dept. of Biotechnology)
Submitted By :-
Deepak Kumar
U.Roll no:- 200450190004
M.Sc Biotechnology (Previous)
Chaudhary Devi Lal University,
(Sirsa)
(A Presentation On)

2. Citric Acid Cycle
Or TCA Cycle
This cycle was discovered by Hans
Krebs in 1937. So, it is also known as
Krebs Cycle.
This cycle is a series of reactions in
mitochondria that bring about the
complete oxidation of acetyl-CoA to CO₂
and liberate hydrogen equivalents, which
ultimately form water.
The Cyclic Sequence of reactions provides
electrons to the transport system, which
reduces oxygen while generating ATP .
The Citric Acid Cycle is the final common
pathway for the oxidation of fuel
molecules – amino acids and
carbohydrates . Most fuel molecule enter
in the cycle as acetyl-CoA.
Fig :- (An outline of TCA Cycle)

3. REACTION STEPS OF THE CITRIC ACID CYCLE
The citric acid cycle proper consists of a total of 8 successive reaction steps,. each of which is
catalyzed by an enzyme. The details of these reactions and those of the enzymes which catalyze
them are given below :
Step 1 : Condensation of acetyl- CoA with oxaloacetate
The citric acid cycle begins with the condensation of an oxaloacetate (4C) and the
acetyl group of acetyl – CoA (2C). Oxaloactete react with acetyl-CoA and H2O to yied
citrate and CoA. This reaction which is an aldol condensation followed by a hydrolysis,
is catalysed by citrate synthase.

4. Step 2 : Isomerization of citrate into isocitrate :
An isomerization reaction, in which water is first removed and the added back,
moves the hydroxl group from one carbon atom to its neighbour. The enzyme
catalyzing this step is aconitase, which convert the citrate into iso-citrate.

5. Step 3 : Oxidative decarboxylation of isocitrate
Isocitrate is oxidised and decarboxylated to α-ketoglutarate . In the first of four
oxidation steps in the cycle, the carbon carrying the hydroxyl group is converted
to a carbonyl group. The immediate product is unstable, losing CO2 while still
bound to the enzyme . The oxidative decarboxylation of iso-citrate is catalyzed
by isocitrate dehydrogenase.

6. Step 4 : Oxidative decarboxylation of a-ketoglutarate
A second oxidative decarbxylation reaction results in the formation of succinyl-
CoA from α-ketoglutarate . α-ketoglutarate dehydrogenase catalyzes this
oxidative step and produces NADH, CO2 and a high energy thioester bond to
CoA.

7. Step 5 : Conversion of succinyl-CoA into succinate
The cleavage of thioester bond of succinyl-CoA is couplded with the
phosphorylation of an ADP or GDP. This step is catalyzed by succinyl-CoA
synthetase . ATP and GTP are energetically equivalent. This is the only step
in the CAC that directly yields a compound with high phosphoryl transfer
potential through a substrate level phosphorylation.

8. Step 6 : Dehydrogenation of succinate to fumarate
In third oxidation step in the cycle, FAD removes two hydrogen atoms from
succinate. The enzyme catalyzing this step, succinate dehydrogenase covert the
succinate into fumerate .

9. Step 7 : Hydration of fumarate to malate
Fumerate is hydrated to form malate in the prensence of fumerase enzyme. Removal
of one molecule of water is occured

10. Step 8 : Dehydrogenation of malate to oxaloacetate
In the last of fourth oxidation steps in the cycle, the carbon carrying the hydroxyl
group is converted to a carbonyl group, regenerating the oxaloacetate needed for
step 1. NAD+ linked malate dehydrogenase catalyzes the oxidation of malate to
oxaloacetate.

11. STOICHIOMETRY OF THE CITRIC ACID CYCLE
A. Overall Balance Sheet:-
We have just seen that one turn of the citric acid cycle
involves 8 enzyme-catalyzed reactions and leads to the
conversion of one mole of acetyl-CoA to CO2 plus H2O.
The net reaction of the cycle may be written as :
CH3CO-SCoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O Acetyl–
CoA
→ 2CO2 + CoA—SH + 3 NADH + FADH2 + GTP +
2H+ Coenzyme A
The ΔGº′ for this overall reaction is –14.3 kcal/mol.

12. B. Energy Yield

13. Control or Regulation of TCA Cycle
1st Control point
• Citrate synthase is
inhibited by ATP, NADH,
acyl- CoA & succinyl-CoA
.
• Level of ATP increase,
less of enzyme is
saturated with acetyl-
CoA and less citrate is
formed.
2nd Control point
• Iso-citrate
dehydrogenase is
activated by ADP &
inhibited by ATP and
NADH.
• NADH act as allosteric
inhibitor which act
directly displacing NAD.
3rd Control point
• α-ketoglutarate
dehydrogenase is
inhibited by succinyl-CoA
& NADH.
• Succinate
dehydrogenase is also
inhibited by malonate.

14. Dual Role Of
TCA Cycle
B)
Anapleortic
Roles
A)
Biosynthetic
Roles

15. ( Biosynthetic Role )
• Oxaloacetate is prescursor for Aspartate.
• α-ketoglutarate can be transaminated to
Glutamate.
•Succinyl-CoA is used for synthesis of heme.
• Mitochondrial citrate is transported to
cytoplasm & it is cleaved into acetyl-CoA to
provide substrate for fatty acid synthesis.

16. ( Anaplerois or Anapleortic Roles )
• Pyruvate caboxylase catalyses conversion of pyruvate to oxaloacetate.
• This is an ATP dependent carboxylation reaction.

17. Fig : Biosynthetic and anaplerotic functions of the citric acid cycle